After getting calls from enough areas, not all, the owner will tell you to meet him / her at the Ferris Wheel in Nimbasa City's Amusement Park. Talk to them to return the item.

Try to call the trainer as much as possible at later times. He / she may not pick up since they're at work, but they will eventually pick up. After enough calls are made, they'll offer to trade Pokemon with you.

The gender and character will depend on the gender of your character. If your character is a male, the owner will be a female, and vice versa. The different trades they offer will depend on the gender of the character. They will take any Pokemon to trade for their rare Pokemon at Level 50.